题目

 Yesterday evening Tammy had just finished her homework ________ her mother asked her to practice playing the piano.   A. while              B. when            C. until           D. before 答案：B 天平两托盘上的烧杯中，分别放有足量的等质量、等溶质质量分数的稀硫酸，调整天平至平衡后，向天平左端烧杯中加入4.4克氧化铁，欲保持天平平衡，应向右端的烧杯中加入   （     ） A、4.8克Mg                  B、5.4克Al C、4.3克Zn                   D、4.5克Ag